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1. Wide and Tunable Bandgap MAPbBr 3−x Cl x Hybrid Perovskites with Enhanced Phase Stability: In Situ Investigation and Photovoltaic Devices

   https://doi.org/10.1002/solr.202000718  

   Tang, Ming-Chun ; Dang, Hoang X. ; Lee, Sehyun ; Barrit, Dounya ; Munir, Rahim ; Wang, Kai ; Li, Ruipeng ; Smilgies, Detlef-M. ; De Wolf, Stefaan ; Kim, Dong-Yu ; et al ( March 2021 , Solar RRL)

   The current understanding of the crystallization, morphology evolution, and phase stability of wide‐bandgap hybrid perovskite thin films is very limited, as much of the community's focus is on lower bandgap systems. Herein, the crystallization behavior and film formation of a wide and tunable bandgap MAPbBr_3−xCl_xsystem are investigated, and its formation and phase stability are contrasted to the classical MAPbI_3−xBr_xcase. A multiprobe in situ characterization approach consisting of synchrotron‐based grazing incidence wide‐angle X‐ray scattering and laboratory‐based time‐resolved UV–Vis absorbance measurements is utilized to show that all wide‐bandgap perovskite compositions of MAPbBr_3−xCl_xstudied (0 < x < 3) crystallize the same way: the perovskite phase forms directly from the colloidal sol state and forms a solid film in the cubic structure. This results in significantly improved alloying and phase stability of these compounds compared with MAPbI_3−xBr_xsystems. The phase transformation pathway is direct and excludes solvated phases, in contrast to methylammonium lead iodide (MAPbI₃). The films benefit from antisolvent dripping to overcome the formation of discontinuous layers and enable device integration. Pin‐hole‐free MAPbBr_3−xCl_xhybrid perovskite thin films with a tunable bandgap are, thus, integrated into working single‐junction solar cell devices and achieve a tunable open‐circuit voltage as high as 1.6 V.

    

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2. All‐Printed Ultrahigh‐Responsivity MoS 2 Nanosheet Photodetectors Enabled by Megasonic Exfoliation

   https://doi.org/10.1002/adma.202203772  

   Kuo, Lidia ; Sangwan, Vinod K. ; Rangnekar, Sonal V. ; Chu, Ting‐Ching ; Lam, David ; Zhu, Zhehao ; Richter, Lee J. ; Li, Ruipeng ; Szydłowska, Beata M. ; Downing, Julia R. ; et al ( July 2022 , Advanced Materials)

   Printed 2D materials, derived from solution‐processed inks, offer scalable and cost‐effective routes to mechanically flexible optoelectronics. With micrometer‐scale control and broad processing latitude, aerosol‐jet printing (AJP) is of particular interest for all‐printed circuits and systems. Here, AJP is utilized to achieve ultrahigh‐responsivity photodetectors consisting of well‐aligned, percolating networks of semiconducting MoS₂nanosheets and graphene electrodes on flexible polyimide substrates. Ultrathin (≈1.2 nm thick) and high‐aspect‐ratio (≈1 μm lateral size) MoS₂nanosheets are obtained by electrochemical intercalation followed by megasonic atomization during AJP, which not only aerosolizes the inks but also further exfoliates the nanosheets. The incorporation of the high‐boiling‐point solvent terpineol into the MoS₂ink is critical for achieving a highly aligned and flat thin‐film morphology following AJP as confirmed by grazing‐incidence wide‐angle X‐ray scattering and atomic force microscopy. Following AJP, curing is achieved with photonic annealing, which yields quasi‐ohmic contacts and photoactive channels with responsivities exceeding 10³ A W⁻¹that outperform previously reported all‐printed visible‐light photodetectors by over three orders of magnitude. Megasonic exfoliation coupled with properly designed AJP ink formulations enables the superlative optoelectronic properties of ultrathin MoS₂nanosheets to be preserved and exploited for the scalable additive manufacturing of mechanically flexible optoelectronics.

    

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3. Bi 2 Se 3 van der Waals Virtual Substrates for II–VI Heterostructures

   https://doi.org/10.1002/pssb.201700275  

   Garcia, Thor Axtmann ; Deligiannakis, Vasilios ; Forrester, Candice ; Levy, Ido ; Tamargo, Maria C. ( August 2017 , physica status solidi (b))

   We report on the growth and characterization of optical quality multiple quantum well structures of Zn_xCd_1−xSe/Zn_xCd_yMg_1−x−ySe on an ultra‐thin Bi₂Se₃/CdTe virtual substrate on c‐plane Al₂O₃(sapphire). Excellent quality highly oriented films grown along the (111) direction were achieved as evidenced by reflection high energy electron diffraction and X‐ray diffraction studies. We also observed room temperature and 77 K photoluminescence emission with peak energies at 77 K of 2.407 eV and linewidths of 56 meV comparable to those achieved on structures grown on InP. Exfoliation of the structures is also possible due to the van der Waals bonding of Bi₂Se₃. Exfoliated (substrate free) films exhibit photoluminescence emission nearly identical to that of the supported film. Additionally, contactless electroreflectance measurements show good agreement with simulations of the multiple quantum well structure as well as evidence of excited state levels. These results open new avenues of research for substrate independent epitaxy and the possibility of ultra‐thin electronics.

    

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4. Electrical and optical properties of nickel-doped Ge2Sb2Te5 films produced by magnetron co-sputtering

   https://doi.org/10.1117/12.2320843  

   Guo, Pengfei ; Sevison, Gary ; Agha, Imad ; Sarangan, Andrew ; Burrow, Joshua ; Sakdinawat, Anne E. ; Attias, André-Jean ; Panchapakesan, Balaji ; Dobisz, Elizabeth A. ( September 2018 , Proceedings of SPIE, Nanoengineering: Fabrication, Properties, Optics, and Devices XV)

   A magnetron co-sputtering system was used for producing nickel-doped Ge2Sb2Te5 (GST-Ni) thin films. The nickel content in the thin film was adjusted by the ratio of the plasma discharge power applied to the GST and nickel targets, as well as a physical shuttering technique to further control the nickel deposition rate. The doping concentration of the film was confirmed using Energy Dispersion Spectroscopy (EDS) technique. Results from a four-point probe measurement indicate that the nickel doping can reduce the resistivity of GST in the amorphous state by nearly three orders of magnitude. The dopant's influence on crystallization behavior was studied by analyzing X-Ray Diffraction (XRD) patterns of the pure GST and GST-Ni at different annealing temperatures. To examine the structural changes due to the nickel dopant, the thin films were investigated with the aid of Raman scattering. Additionally, we extracted the optical constants for both the amorphous and crystalline states of undoped-GST and GST-Ni films by ellipsometry. The results indicate that at low doping concentrations nickel does not appreciably affect the optical constants, but dramatically improves the electrical conductivity. Therefore, nickel-doping of GST a viable method for designing optical devices for lower operating voltages at higher switching speeds. 

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5. (Digital Presentation) Calculation of the Energy Band Diagram and Estimation of Electronic Transport Parameters of Metastable Amorphous Ge 2 Sb 2 Te 5

   https://doi.org/10.1149/10801.0029ecst  

   Kashem, Md Tashfiq ; Muneer, Sadid ; Adnane, Lhacene ; Dirisaglik, Faruk ; Gokirmak, Ali ; Silva, Helena ( May 2022 , ECS Transactions)

   We calculate critical electronic conduction parameters of the amorphous phase of Ge 2 Sb 2 Te 5 (GST), a common material used in phase change memory. We estimate the room temperature bandgap of metastable amorphous GST to be E g (300K) = 1.84 eV based on a temperature dependent energy band model. We estimate the free carrier concentration at the melting temperature utilizing the latent heat of fusion to be 1.47 x 10 22 cm -3 . Using the thin film melt resistivity, we calculate the carrier mobility at melting point as 0.187 cm 2 /V-s. Assuming that metastable amorphous GST is a supercooled liquid with bipolar conduction, we compute the total carrier concentration as a function of temperature and estimate the room temperature free carrier concentration as p(300K) ≈ n(300K) = 1.69×10 17 cm -3 . Free electrons and holes are expected to recombine over time and the stable (drifted) amorphous GST is estimated to have p-type conduction with p(300K) ≈ 6×10 16 cm -3 . 

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